Induction of unscheduled DNA synthesis in hairless mouse epidermis by ultraviolet light.

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Abstract
The activity of ultraviolet (UV) light to induce unscheduled DNA synthesis (UDS) was investigated in hairless mouse epidermis by means of an in vivo-in vitro assay using a liquid scintillation counting method. Groups of three to five 8-week-old female hairless mice were irradiated with UV-B or UV-A, then skin samples were taken and cultured individually in medium containing [3H]thymidine with or without hydroxyurea (HU) for 2 hr. DNA of the epidermis was extracted, and incorporation of [3H]thymidine and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS was judged in terms of the UDS index [(the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. UV-B increased the UDS index 1 hr after irradiation of 500 J/m2, which corresponds to approximately 1 minimal erythema dose or 1 minimal edema dose, and showed a dose-dependent increase up to 17-fold in the UDS index at irradiation doses of 500 to 2,000 J/m2. In a time-course study, UV-B also increased replicative DNA synthesis (RDS) 48 hr after irradiation at 1,000 J/m2. On the other hand, UV-A did not increase the UDS index at irradiation doses of 2 x 10(5) to 8 x 10(5) J/m2. These results show that induction of UDS by UV irradiation depends on wavelength and an increase of RDS in the epidermis exposed to UV-B irradiation appears after induction of UDS.

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